Of same place



(No Model!) W. W. DEAN. LOCAL TRANSMITTER CIRCUIT FOR TELEPHCNES.

No. 549,477. Patented Nov. 5, 1895.

' William 1d Dean,

UNITED STATES PATENT OEETCE.

\VILLIAM W. DEAN, OF ST. LOUIS, MISSOURI, ASSIGNOR TO THE BELL TELEPHONECOMPANY OF MISSOURI, OF SAME PLACE.

LOCAL TRANSMITTER-CIRCUIT FOR TELEPHONES.

SPECIFICATION forming part of Letters Patent No. 549,477, dated November5, 1895.

Application filed March 2 9 1 8 9 5.

vide improved means for producing in a local transmitter-circuitvoice-currents adapted to be propagated over a telephone-line to adistant receiver.

In an application for Letters Patent filed February 21, 1895, Serial No.539,209, I have described a local transmitter-circuit comprising aclosed circuit including the microphone and the primary winding of aninductioncoil, the local transmitter-circuit being'con- I nected withthe telephone-line at two points, to thus include the localtransmitter-circuit in the line-circuit in two parallel branches.Current from a charging-battery connected with the line thus traversesthe local circuit in two parallel paths, and the operation of themicrophone in one of the paths causes a variation of the current in thetwo paths of the local circuit. I have also illustrated in saidapplication a form of transmitter-circuit in which the microphone andthe primary winding of one induction-coil are included in one of theparallel branches, while the primary winding of a second induction-coilis included in a second branch, the actuation of the microphone thusproducing opposite effects in the two induction-coils. The secondarywindings of the induction-coils are included in series in thetelephone-line, the voltages of the two coils being thus added.

In accordance with the present invention I provide two microphones, onein each of the parallel circuits, the two microphones being connectedwith a common diaphragm or sounding-board, so that as one microphoneincreases the resistance through its circuit the other microphonedecreases the resistance through its circuit, the two microphones thusacting always in opposite directions.

Serial No. 543,662. (No model.)

The result is to greatly increase the change of current through the twoparallel paths. Thus movement of one microphone to increase theresistance of its path causes an increased current through the otherpath but the microphone in this second path is simul taneously actuatedto decrease the resistance through the second path. The current flowingthrough the second path is thus increased through two causesfirst, theincrease of resistance in the first path, and, second, the decrease ofthe resistance in the second path.

I Will describe my invention more in particular by reference to theaccompanying drawings, in which- Figure 1 is a diagrammatic viewillustrating my invention. Fig. 2 is a view of the dupleX microphonewhich I employ in practicing my invention.

Like letters refer to like parts throughout the several figures.

In Fig. 1 I have illustrated a local transmitter-circuit in heavy lines,the circuit comprising two branches a and b; In the branch at isprovided a microphone a" and the primary winding a of an induction-coil.In the branch I) is provided a microphone Z) and the primarywindingb ofan induction-coil. The secondary windings a and b of the induction-coilsare included in series in the telephone-circuit, being connected byconductors c and c with contact points 0 0 against which the switch-hookc rests when the telephone is removed therefrom to close circuit throughthe limbs (Z (Z of the telephone line, which extends to the centralstation and terminates in the line-springs d d of a springjack. Theline-spring d normally rests upon a contact d, connected through anindicator d and battery e to ground. At the subscribers station isprovided a contact f, connected to ground through a bell f. The hook cnormally rests upon-contact f to close the circuit of the battery 6through indicator 61 When the subscriber removes his telephone from itshook, the hook c breaks contact with the contact f, thus opening thecircuit of the battery ethrough indicator d whereby the signal forconnection is conveyed to the operator, who connects the line of thecalling subscriber with the line of the called subscriber by means ofplugs g g, inserted in the spring-jacks, as illustrated. In one of thestrands of the cord-connectors are provided clearing-out indicators h h,and between the strands is provided abridge 72 containing aretardation-coil 71*, from the center of which extends a. conductor 72"to the 'pole of the battery c.

The receiver 1'. of subscriber A is provided in a bridge between the twoconductors c c. The local transmitter-0ircuit of subscriber A isconnected to ground at a point I)" and is connected at the point U withconductors c c. The apparatus and connections of substation B aresimilarto those of sub-station A and have been indicated bycorresponding reference-letters. I have illustrated the receiver 'i ofsubstation B asincluded in series with the secondaries of theinduction-coils, instead of. in parallel therewith, as shown insub-station A. The point I) of the local transmitter-circuit atsub-station B is shown connected to ground, while the point 11 isconnected to the middle of a retardation-coil 7:, connected in abridgebetween the conductors c c. I omit further description of the telephonesystem illustrated, asthis forms no part of the present invention andhas been fully described in the application above referred to.

It will be noted that current from battery c passes to the point 11'' ofthe local transmitter-circuit of sub-station A over the two limbs of thetelephone-line in parallel, the current then passing in parallel throughthe two branches a and b of the localcireuit to ground. As thesubscriber at sub-station A talks into his transmitter, the resistancethrough the microphone to is increased simultaneously with the decreaseof resistance through the microphone b. The current flowing through thebranch (L is thus decreased, due to the increase of resistance throughthe microphone a, and, further, to the decrease of resistance throughthe branch 1). The current through branch Z) is increased, due to thedecrease of resistance through the microphone Z) and the increase ofresistance through the branch 0. Changes of current-strength are thusproduced in the primaries (L2 b of the induction coils,which inducecurrents in the secondaries (1. 0, the secondaries being wound in such adirection that the voltages of the secondary windings will be added.

In Fig. 2 I have illustrated a form of. transmitter comprising twomicrophones operated by one diaphragm. Upon the supports I and m areprovided carbon buttons l and m, the ends thereof being opposed. Betweenthe opposed faces of the carbons is provided a pin n,carried upon abracket n,mounted upon the diaphragm 0. As the diaphragm \jibrates thecontact between the pin 72. and the carbon buttons Z and m is varied,the pin making more intimate contact with button m and a simultaneouslyweakened contact with button Z, and vice versa. The contact-pin n andbutton l may be included in one of the branches of the localtransmitter-0ircuit, as the branch (I, while the pin 02. and button mmay be included in the other branch 7) of the local circuit.

Having thus described my invention, what I claim as new, and desire tosecure by Letters Patent, is

1. The combination with a local transmitter circuit comprising twoparallel branches, of two induction coils having their primariesincluded one in each of the branches of said transmitter circuit andtheir secondaries in the telephone line, a microphone included in eachof the branches of said transmitter circuit and means for simultaneouslyincreasing the resistance through one of the microphones as theresistance through the other microphone is decreased, and a sourceotcurrent connected with said telephone line adapt ed to direct currentthrough said branches in parallel; substantially as described.

The combination with a local transmitter circuit comprising two parallelbranches, of two induction coils having their primaries included one ineach of the branches of said transmitter circuit and their secondariesincluded in series in the telephone line, a microphone included in eachof the branches of said transmitter circuit, means for increasing theresistance through one of the microphones simultaneously with a decreaseof resistance in the other microphone, and a source of electricityconnected with said telephone line adapted to direct current throughsaid branches in parallel; substantially as described.

In witness whereof I hereunto subscribe my name this 25th day of March,A. l). 1895.

'31. \V. DEAN.

\Vitnesses:

EUGENE DEVINE, THos. E. FLAHERTY.

